1. Field of the Invention
The present invention relates to a method for determining the reference phase in a radio communication system that uses orthogonal M-ary modulation, and a coherent detection method that uses the same.
2. Description of the Related Art
Code division multiple access (CDMA) systems are gaining attention as next-generation mobile communication systems, and a standard system, called IS-95, already exists in the United States. As means of infrastructure building, CDMA systems based on a semi-fixed mobile communication system called a wireless local loop (WLL) could also be put to use.
This is a CDMA system with a chip rate of 1.2288 Mcps proposed by Qualcomm, Inc. of the United States, the downlink of which uses a coherent detection system, which utilizes a pilot extrapolation signal. Conversely, the uplink uses an non-coherent detection system, which utilizes orthogonal M-ary modulation.
As for the non-coherent detection system, which uses orthogonal M-ary modulation in the uplink in this IS-95 standard system, by converting the amplitude signal to power at the receiving end, phase errors caused by fading and the like are removed to prevent degradation. However, error rate characteristics deteriorate more during non-coherent detection than during coherent detection.
Therefore, an object of the present invention is to provide a method for determining the received reference phase of an orthogonal M-ary modulated radio communication system, and a demodulation method that uses the same, so as to enable the enhancement of the performance of the uplink in an IS-95 standard system.
A further object of the present invention is to provide a coherent detection system for orthogonal M-ary modulated CDMA radio communications with broader applications. Another object of the present invention is to provide a method that makes it possible to determine the received reference phase at the receiving end when performing orthogonal M-ary modulation for orthogonal coding as a general application.
Other objects of the present invention are clarified in the description of the embodiments of the present invention below.
A method for determining the reference phase of a radio communication system, which achieves the above-cited tasks of the present invention, is based on determining the correlation values of an in-phase component and quadrature component corresponding to respective Hadamard matrices resulting from fast Hadamard transform or inverse Hadamard transform of a signal, which possesses this in-phase component and quadrature component, and which is sent from the sending end by phase shift keying an orthogonal M-ary code; computing the sum of the squares of the respective correlation values of this in-phase component and quadrature component; and selecting either the fast Hadamard transform or inverse Hadamard transform output, which outputs a maximum value of the computed sum of the squares.
Such a radio communication system uses a reference phase determination method, and comprises a plurality of base stations and a plurality of terminals.
A demodulation method of a radio communication system, in which a plurality of base stations and terminals are connected via spread spectrum communications, and which uses a code division multiple access system, which performs orthogonal M-ary modulation on a data signal, followed by spread modulation, determines an in-phase component and an quadrature component by de-spreading a reception signal, and determines the correlation values of this in-phase component and quadrature component, which correspond to respective Hadamard matrices resulting from either a fast Hadamard transform or an inverse Hadamard transform performed on this in-phase component and quadrature component. Further, the orthogonal M-ary modulated radio communication system and the base stations utilized therein are characterized in computing the sum of the squares of the respective correlation values of the in-phase component and quadrature component; selecting either a fast Hadamard transform or inverse Hadamard transform output, which outputs the maximum value of the computed sum of the squares; carrying out coherent detection by performing complex multiplication of the selected fast Hadamard transform or inverse Hadamard transform output and the in-phase component and quadrature component determined by this de-spreading; determining the correlation value by the fast Hadamard transform or inverse Hadamard transform of the coherent detection output; and converting an Hadamard matrix, which corresponds to the maximum value of the determined correlation value, to a data signal.
The orthogonal M-ary modulated radio communication system and the base stations are further characterized in utilizing a moving average filter to equalize this fast Hadamard transform or inverse Hadamard transform output, which outputs the maximum value of the computed sum of the squares described above.
The orthogonal M-ary modulated radio communication system and the base stations are also characterized in utilizing a primary linear interpolation filter to equalize a fast Hadamard transform or inverse Hadamard transform output, which outputs the maximum value of the computed sum of the squares described above.
The orthogonal M-ary modulated radio communication system and the base stations are also are characterized in that primary linear interpolation is also performed on the output of the above-cited moving average filter in the above-described.